The invention relates to a process for the isolation of highly enriched to very pure 2,4- and 2,6-dinitrotoluenes (DNT) from mixtures containing these isomers.
It is known that isomer mixtures consisting of 2,4- and 2,6-DNT in a ratio of about 65:35 to about 85:15 are obtained in the nitration of toluene or o-nitrotoluene; small amounts of in each case about 0.1 to 1% of other isomers (2,5-, 2,3- and 3,4-DNT) are also formed.
In the context of the invention, isolation of the isomers mentioned is to be understood as meaning the obtaining of at least 95% pure isomers, preferably about 99% pure isomers of DNT. 2,6-DNT in particular is obtained in an exceptionally high purity of in many cases 99.5% or more.
Separation of the isomer mixtures which are available industrially in order to isolate enriched or pure isomers is extremely difficult. Processes for separation by various methods of adsorption, for example on zeolites, have been disclosed in the literature (U.S. Pat. No. 4,642,397), it being necessary for the isomer adsorbed selectively onto the zeolite to be desorbed again using another solvent. These processes accordingly require large amounts of organic solvents or mixtures of such solvents (such as, for example, butanol/toluene), which, for reasons of protection of the environment, must be separated off, purified and recycled to the process at great expense. Furthermore, only very dilute solutions can be separated in this manner (according to Example 1 of U.S. Pat. No. 4,642,397 0.5 g of 2,4-DNT and 0.5 g of 2,6-DNT in 27.3 g of mesitylene), which pushes up the volumes of organic solvents required further. Due to these limitations alone, the process described is very expensive and economic utilisation on an industrial scale is prevented.
The separation of 2,4- and 2,6-DNT by melt crystallisation is furthermore known (C.A. 87 (1977), 134 189 y). However (probably because of the small differences in melting points), the separation is not very efficient (melting point of 2,4-DNT: 71.degree. C.; melting point of 2,6-DNT: 69.degree. C.), so that high purities can be achieved only by repeated crystallisation and with the associated high technical expenditure. Furthermore, in this process only the 2,4-DNT can be separated off effectively from the isomer mixture obtainable on an industrial scale; the 2,6-DNT remains enriched in the residue. The technical object imposed of preparing pure 2,6-DNT therefore cannot be achieved with this process.
Attempts to separate 2,4- and 2,6-DNT by treatment with organic solvents have furthermore been disclosed.
In the case of U.S. Pat. No. Patent 3,949,008 (extraction with C5-C8-alkanes, specifically with hexane), only traces of secondary isomers could be removed, while the main isomers could not be separated. In the case of recrystallisation from ethanol or C.sub.2 HCl.sub.3 (C.A. 95 (1981), 80 328 n), it is only possible to prepare 2,4-DNT in a pure form.
The separation of pure 2,6-DNT from isomer mixtures according to U.S. Pat. No. 2,765,348 is achieved only by recrystallisation of solvents which are unacceptable for industrial hygiene reasons, such as N,N-dimethylaniline or toluidine. In this process, filtration has to be carried out at low temperatures (for example at -10.degree. C.), which requires a high technical expenditure. Since the desired product is obtained in only a poor yield (for example 53%) and moderate purity (for example 97%), this process is not economical to carry out.
Only the 2,4-isomer is obtained in a pure form even by crystallisation from DNT isomer mixtures in sulphuric acid (EP 66,202).
Finally, attempts have been made to treat DNT mixtures with reducing agents, such as Na.sub.2 S.sub.2 (U.S. Pat. No. 3,931,347), Na.sub.2 SO.sub.3 (JP 45/9169 (1970); cited in C.A. 73 (1970), 14 438 t) or N.sub.2 H.sub.4 (JP 56/142,245 (1981); cited in C.A. 96 (1982), 85 228 k). In these processes, small amounts of undesirable position isomers are converted into watersoluble products, but the desired 2,4- and 2,6-isomers are not separated.
The preparation of pure 2,6-DNT has indeed already been described, but is carried out by cumbersome routes, that is to say
a) by diazotisation and reduction of 2,6-dinitro-4-amino-toluene (Can. J. Chem. 37 (1959), 2073) or
b) by heating 2,6-dinitro-phenylacetic acid (Ann. 379 (1911), 152, in particular 181).
Since 2,6-DNT in particular is a sought-after intermediate product for the preparation of dyestuffs and pigments, pharmaceuticals, plant protection agents, polyurethanes and cosmetic products, it was therefore to be assumed that a simple preparation possibility or possibility for isolating the abovementioned products experts had considered such simple methods possible.